Free-viewing of stereoscopic displays such as photographs is an enjoyable experience. A person with satisfactory vision from both eyes stands several feet away from two stereoscopic images such as photographs, a left and right. The person, trained in free-viewing sets his/her left eye to focus onto the right photograph (which is indeed the left image) while setting their right eye to focus onto the left photograph (which is actually the right image). When properly done, the person sees three images. The middle image appearing to have depth, or a third dimension. Stereoscopic photographs are photographs taken of the same image with small spatial and angular displacement alike images seen from a left eye and a right eye.
For free-viewing to happen muscles of the eyes have been trained to shift the line of sight from the object of their vision, the left eye to cross over the center towards the right and the right eye to cross over the center towards the left. Free-viewing superimposes the right image from the left eye onto the left image from the right eye and in focus. The mind seeing two photographs identical except for small spatial and angular displacements superimposed upon themselves, interprets the center image as one having the depth of an object with three dimensions. Once the free-viewing of the image has occurred, the mind controlling the muscles of the eyes maintains the three dimensional position of the image.
Free-viewing is a memorable experience albeit difficult to achieve since the eyes must be kept at a convergence greater than that dictated by the normal focus. To attain this convergence the focus must be kept on a point beyond the convergent point. This focus is impossible to achieve for some people and produces discomfort in others. The voluntary muscles are not sufficiently developed to overcome the natural impulse to focus onto separate objects or to operate outside their normal range.
Stereoscopic viewers are old and well known in the prior art of optics. Many of the basic concepts of stereoscopic viewing were derived in the late 1800's and early 1900's. However, this area of optics has been largely ignored in recent times.
New developments in this area have mainly been confined to the projection of stereoscopic images or the actual photography or development of superimposed images using modified stationary and motion cameras. the U.S. Pat. No. 3,891,303 to Barquero shows an adaptor to be used with cameras to produce stereoscopic image. Barquero shows a very effective way of producing a central superimposed image by using prisms before the viewing lenses and using screens in front of the prisms to block out secondary image. the U.S. Pat. No. 3,039,358 to Vierling uses Barquero's concept and incorporates the prisms and screens integrally within a camera.
The re-emergence of 3-dimensional projection has been given much attention in recent years and has not been confined to still photography. The development of this technology for motion pictures, television, and computer images has been growing at a rapid pace. However, stereoscopic viewers have remained largely undeveloped and no significant improvements have been made in viewing devices in many years.
The device disclosed in the U.S. Pat. No. 262,846 to Stevens was a standard type of stereoscopic viewing device used well into the 1900's. This device has some severe limitations. The device requires a specific size photograph, three or four major adjustments were needed to view the 3-dimensional image, and two sets of image blocking screens were needed to block undesired images. the U.S. Pat. No. 3,502,390 to Gilmore shows a much simpler device to view 3-dimensional images of x-rays. However, outer walls and a central partition are needed to force the eyes to see the superimposed image. These walls force the pictures or x-rays to be placed at a predetermined distance and remain a standard size.
One viewing device that went a long way to correcting the deficiencies of the above noted devices is the device disclosed in the U.S. Pat. No. 1,440,457 to Girsdansky. This device divorces the viewing lenses from the viewing screen(s). This was a significant breakthrough in 3-D viewing. By arranging the prisms in a manner similar to the projectors of Barquero and Vierling, Girsdansky's viewing device enables a viewer to induce free-viewing and see superimposed images of any size and at great distances for applications in auditoriums, museums and theaters.
But Girsdansky's device has several disadvantages which have remained unimproved on for more than sixty years. First, the blocking screens require both a longitudinal and a lateral adjustment for the viewer to focus properly on the superimposed image. Second, the blocking screens move laterally together and apart at concurrent distances. This does not allow for complete focusing adjustment for all types of persons who may need one blocking screen moved further in than the opposite blocking screen. Third, the openings within the viewer limit the width of the pictures being viewed; thus, to view wide motion picture screens on would have to sit well back in the theater to see the entire screen. Finally, the holder of Girsdansky's device is generally bulky and would be expensive to manufacture requiring many different parts.